Generally, conference circuits are used to combine speech signals and to generate a resultant signal which in effect, represents the sum of the speech input signals. Delta modulation is a pulse modulation technique in which a continuous analog signal is converted into a digital bit stream for transmission via a selected medium. Conference circuits for combining delta modulated speech signals are well known in the art. In one typical prior art conference circuit, each of the delta modulated speech signals in digital form is decoded into an analog signal. All of these analog signals are linearly summed to form a resultant analog signal which is encoded into a resultant delta modulated signal. Another pulse modulation technique is pulse code modulation. In one typical prior art conference circuit, pulse code modulated speech signals in digital form are directly added by a digital adder to form a resultant pulse code modulated signal. However, a logic circuit for combining pulse code modulated signals is not suited for combining delta modulated signals. A pulse code modulated signal is comprised of groups of pulses or bits, typically eight in number, and each group represents the magnitude of an analog signal when sampled. Pulse code modulated signals can be directly added to represent the linear sum of the magnitudes of corresponding analog signals. By comparison, in a delta modulated signal, each bit only represents the direction of change in the magnitude of an analog signal; thus, the result obtained by the binary addition of delta modulated signals would not represent the linear sum of the magnitudes of corresponding analog signals.
Prior conference circuits for combining delta modulated speech signals suffer from the problem of requiring a decoder circuit for converting each delta modulated speech signal to an analog signal prior to summation and an encoder circuit for converting the resultant analog signal to a resultant delta modulated signal. The use of these decoder and encoder circuits results in increased bit error probability, increased sampling noise, and increased cost.